Disk files are information storage devices which utilize a rotatable disk with concentric data tracks containing the information, a head for reading or writing data onto the various tracks, and an actuator connected by a support arm assembly to the head for moving the head to the desired track and maintaining it over the track centerline during read or write operations. The movement of the head to a desired track is referred to as track accessing or "seeking", while the maintaining of the head over the centerline of the desired track during a read or write operation is referred to as track "following".
The actuator is typically a "voice coil motor" (VCM) which comprises a coil movable through the magnetic field of a permanent magnetic stator. The application of current to the VCM causes the coil, and thus the attached head, to move rapidly. The acceleration of the coil is proportional to the applied current, so that ideally there is no current to the coil if the head is perfectly stationary over a desired track.
In disk files which have a relatively high density of data tracks on the disk, it is necessary to incorporate a servo control system to maintain the head precisely over the centerline of the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. The servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disk is used) is demodulated to generate a position error signal (PES) which is an indication of the position error of the head away from the nearest track centerline.
In a disk file digital servo control system, a microprocessor utilizes a control signal algorithm to calculate a digital control signal based upon the digital values of certain state variables such as PES, VCM current and head velocity. The digital control signal is converted to an analog signal and amplified to provide input current to the VCM. Such a digital servo control system is described in U.S. Pat. No. 4,412,161, wherein the digital control signal is calculated recursively from prior control signals and prior values of the PES.
A recent development in digital disk file servo control systems, as described in assignee's U.S. Pat. No. 4,679,103, is a digital servo control system which, as part of the computation of the control signal to the actuator, makes use of a state estimator algorithm to estimate the position and velocity of the head. In this type of system, a microprocessor receives, at discrete sample times, digital values corresponding to the PES and the actuator input current, and computes, through the use of the state estimator algorithm, a digital control signal. The digital control signal is then converted to an analog signal and amplified to provide a new actuator input current. The method of estimating the state of the physical plant to be controlled in such a digital servo control system requires the use of estimator constants, the derivation of which is described in Digital Control of Dynamic Systems, Franklin and Powell, Addison-Wesley Publishing Co. (1983), chapter 6, pages 131-139. In the case of a disk file, these estimator constants are dependent upon the values of certain physical parameters of the disk file, such as the mass of the coil and head/arm assembly, the actuator force constant (the force applied to the coil per unit of input current), the gain of the VCM power amplifier, the PES gain and the time between PES samples (the PES sampling time).
In digital servo control systems, the PES is available to the microprocessor at a much higher rate than the rate at which the microprocessor is capable of generating the control signal. The time for the microprocessor to compute successive control signals during track seeking is substantially greater than the time to compute successive control signals during track following because of the increased complexity of the seek control signal algorithm. Thus, even though the PES is available during track following at a much higher rate than the rate at which the control signal is generated, the digital servo control system cannot take advantage of the availability of the PES because it is constrained by the relatively slow PES sampling rate required during track seeking. During track following, the PES does not remain precisely at zero (and thus the head does not remain precisely over the track centerline). Rather, the PES experiences nonrepeatable runout caused by such factors as drive motor bearing inaccuracy, disk media defects and PES quantization errors. In order to optimize the disk file operation during track following, it is therefore desirable to reduce the effect of nonrepeatable runout by substantially increasing the PES sampling rate.